The identification, analysis and- monitoring of biological analytes (such as polypeptides, polynucleotides, polysaccharides and the like) or environmental analytes (such as pesticides, biowarfare agents, food contaminants and the like) has become increasingly important for research and industrial applications. Conventionally, analyte detection systems are based on analyte-specific binding between an analyte and an analyte-binding receptor. Such systems typically require complex multicomponent detection systems (such as ELISA sandwich assays) or electrochemical detection systems, or require that both the analyte and the receptor are labeled with detection molecules (for example fluorescence resonance energy transfer or FRET systems).
One method for detecting analyte-binding agent interactions involves a solid phase format employing a reporter labeled analyte-binding agent whose binding to or release from a solid surface is dependent on the presence of analyte. In a typical solid-phase sandwich type assay, for example, the analyte to be measured is an analyte with two or more binding sites, allowing analyte binding both to a receptor carried on a solid surface, and to a reporter-labeled second receptor. The presence of analyte is detected based on the presence of the reporter bound to the solid surface.
A variety of devices for detecting analyte/receptor interactions are also known. The most basic of these are purely chemical/enzymatic assays in which the presence or amount of analyte is detected by measuring or quantitating a detectable reaction product, such as gold immunoparticles. Analyte/receptor interactions can also be detected and quantitated by radiolabel assays. Quantitative binding assays of this type involve two separate components: a reaction substrate, e.g., a solid-phase test strip and a separate reader or detector device, such as a scintillation counter or spectrophotometer. The substrate is generally unsuited to multiple assays, or to miniaturization, for handling multiple analyte assays from a small amount of body fluid sample.
Biosensor devices integrate the assay substrate and detector surface into a single device. One general type of biosensor employs an electrode surface in combination with current or impedance measuring elements for detecting a change in current or impedance in response to the presence of a ligand-receptor binding event. This type of biosensor is disclosed, for example, in U.S. Pat. No. 5,567,301. Gravimetric biosensors employ a piezoelectric crystal to generate a surface acoustic wave whose frequency, wavelength and/or resonance state are sensitive to surface mass on the crystal surface. The shift in acoustic wave properties is therefore indicative of a change in surface mass, e.g.; due to a ligand-receptor binding event. U.S. Pat. Nos. 5,478,756 and 4,789,804 describe gravimetric biosensors of this type. Biosensors based on surface plasmon resonance (SPR) effects have also been proposed, for example, in U.S. Pat. Nos. 5,485,277 and 5,492,840. These devices exploit the shift in SPR surface reflection angle that occurs with perturbations, e.g., binding events, at the SPR interface. Finally, a variety of biosensors that utilize changes in optical properties at a biosensor surface are known, e.g., U.S. Pat. No. 5,268,305.
All of the above analyte detection systems are characterized by the requirement for a secondary detection system to monitor interactions between the analyte and the receptor. A need still exists for a direct, homogeneous assay for analyte detection, i.e., one that may be used in living cells, which will be more versatile in terms of the range of applications and devices with which it can be used.